<p>A prototyping approach that uses modularity and reusable components is described that enables large real-time systems to be prototyped rapidly. It combines a computational model tailored for real-time systems with a high-level prototyping language (the Prototype System Description Language), a systematic design method for rapid prototype construction, and an automated prototyping environment. The method meets the following requirements: the prototype satisfies its requirements and is traceable to them; the prototype is easy to modify; and the prototype code is easy to read and analyze.</p>